The present invention relates to a scroll-type compressor for compressing helium gas, and, more particularly, to a scroll-type compressor having an oil injection mechanism for injecting a large amount of oil into compression chambers of the compressor for cooling the helium gas under compression, as well as the compressor itself.
A screw compressor of the aforementioned type is known as having an oil injection system wherein an oil of a high pressure and cooled by an oil cooler is injected into the compression chamber through an oil injection port, for cooling the compressor and the gas under compression.
In, for example, specification of the U.S. Pat. No. 4,475,360 a scroll-type compressor intended for use in an air conditioner is proposed, with the compressor having a gas injection system in which a gaseous refrigerant, extracted from an upper portion of a gas-liquid separator of a refrigeration cycle, is injected into spaces of the compression chamber in the compression phase, in order to enhance the cooling or heating power of the air conditioner.
In the case of scroll-type compressors, particularly those used for the compression of helium gas, it is desirable to inject a cooling oil into the compression chamber so as to cool the helium gas under compression and also the compressor, otherwise the compressor is excessively heated by the heat generated by the compressed helium gas.
The gas injection system described above, however, cannot be directly applied to the injection of oil in the scroll-type compressors for helium gas. Namely, in the above described gas injection system, the gas injection port through which the gaseous refrigerant is injected into the space of compression chamber has a diameter smaller than a thickness of a scroll wrap which slides on a scroll end plate in which the gas injection port is opened. Consequently, the gas injection port is intermittently blocked by the axial end surface of the scroll wrap, although the period of blockage of the oil is very short. This inconveniently causes a phenomenon similar to a so-called liquid hammering action, resulting in a pressure pulsation of the oil confined in the oil injection port. The amplitude of the pressure pulsation becomes greater as the oil injection rate is increased, causing a tremendous vibration of the piping connected to the oil injection port, seriously affecting the reliability of the compressor as a whole.
On the other hand, a small sized oil injection port inevitably limits the possible rate of oil injection, failing to provide the required cooling effect under certain operating conditions of the compressor due to shortage of the cooling oil.
It is to be noted also that the injection port of the known injection system is positioned such as to open into the compression chamber only after the compression space thereof has been shut off from the suction port, so that the cooling oil cannot effectively cool the drawn gas which has been heated in the suction stage by the heat of the compressor, with a result that the performance of the compressor is undesireably impaired .